In contrast to two-stroke engines, in four-stroke spark ignition engines and diesel engines, the Carnot type cyclic process is controlled with the aid of gas-exchange valves. These valves were earlier opened by means of a camshaft rotating at half rpm via rocker arms in the rhythm of the work strokes and were again closed by means of spring mechanisms operating as a clamping device.
Because of the relatively large mass of the rocker arms, which move up and down in the rhythm of the strokes, these rocker arms have been replaced in high-speed engines more and more by valve rods drivable by an above-lying camshaft.
All mechanically operating valve controls, which operate via cams, operate more or less sluggishly and do not permit any variable influencing of the control. For this reason, various electromagnetically operating valve controls have been suggested.
Electromagnetically operating valve controls of this kind have, as a rule, an electromagnetic control device for opening the valve and a clamping device for closing the valve. The clamping device is configured as a mechanical spring. Without going into details of configurations of this kind, reference can be made, for example, to the publications: German Patents 3,024,109 and 3,513,103 as well as PCT applications WO 95/00787 and WO 95/00959.
Furthermore, valve control devices have been suggested wherein the basic functions continue to be mechanically executed and additional functions, for example, different control times and like variables are realized, however, with electromagnetic ancillary devices (see, for example, EP 0,312,216).
The disadvantage of the above-mentioned construction principles is essentially that one does not do without the mechanical actuators which are controllable only to a limited extent and, for reasons of safety, one cannot do without them. If one would do without the mechanical components and especially the return springs operating as a clamping device, then, in the case of a loss of power, a collision between piston base and valve plate could not be avoided.